Column widths for picture segmentation

ABSTRACT

Systems and methods for picture segmentation through columns and slices in video encoding and decoding. A video picture is divided into a plurality of columns, each column covering only a part of the video picture in a horizontal dimension. All coded tree blocks (“CTBs”) belonging to a slice may belong to one or more columns. The columns may be used to break the same or different prediction or in-loop filtering mechanisms of the video coding, and the CTB scan order used for encoding and/or decoding may be local to a column. Column widths may be indicated in a parameter set and/or may be adjusted at the slice level. At the decoder, column width may be parsed from the bitstream, and slice decoding may occur in one or more columns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/336,675, filed Dec. 23, 2011, which claims the benefit of U.S.Provisional Application No. 61/472,569, filed Dec. 28, 2010, thecontents of each of which are incorporated herein by reference.

FIELD

Embodiments of the invention relate to video coding, and morespecifically to the segmentation of a coded picture into one or morecolumns and slices.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, video cameras, digital recording devices,video gaming devices, video game consoles, cellular or satellite radiotelephones, and the like. Digital video devices may implement videocompression techniques, such as those described in standards likeMPEG-2, MPEG-4, both available from the International Organization forStandardization (“ISO”) 1, ch. de la Voie-Creuse, Case postale 56,CH-1211 Geneva 20, Switzerland, or www.iso.org, or ITU-T H.264/MPEG-4,Part 10, Advanced Video Coding (“AVC”), available from the InternationalTelecommunication Union (“ITU”), Place de Nations, CH-1211 Geneva 20,Switzerland or www.itu.int, each of which is incorporated herein byreference in their entirety, or according to other standard ornon-standard specifications, to encode and/or decode digital videoinformation efficiently.

A video encoder can receive uncoded video information for processing inany suitable format, which may be a digital format conforming to ITU-RBT 601 (available from the International Telecommunications Union, Placedes Nations, 1211 Geneva 20, Switzerland, www.itu.int, and which isincorporated herein by reference in its entirety) or in some otherdigital format. The uncoded video may be organized both spatially intopixel values arranged in one or more two-dimensional matrices as well astemporally into a series of uncoded pictures, with each uncoded picturecomprising one or more of the above-mentioned two-dimensional matricesofpixel values. Further, each pixel may comprise a number of separatecomponents used to represent color in digital format. One common formatfor uncoded video that is input to a video encoder has, for each groupof four pixels, four luminance samples which contain informationregarding the brightness/lightness or darkness of the pixels, and twochrominance samples which contain color information (e.g., YCrCb 4:2:0).

One function of video encoders is to translate (more generally“transform”) uncoded pictures into a bitstream, packet stream, NAL unitstream, or other suitable transmission format (all referred to as“bitstream” henceforth), with goals such as reducing the amount ofredundancy encoded into the bitstream to thereby increase transmissionrates, increasing the resilience of the bitstream to suppress bit errorsor packet erasures that may occur during transmission (collectivelyknown as “error resilience”), or other application-specific goals.Embodiments of the present invention provided for at least one of theremoval or reduction of redundancy, the increase in error resilience,and implementability of video encoders and/or associated decoders inparallel processing architectures.

One function of video decoders is to receive as its input a coded videoin the form of a bitstream that may have been produced by a videoencoder conforming to the same video compression standard. The videoencoder then translates (more generally “transforms”) the received codedbitstream into uncoded video information that may be displayed, stored,or otherwise handled.

Both video encoders and video decoders may be implemented using hardwareand/or software configuration, including combinations of both hardwareand software. Implementations of either or both may include the use ofprogrammable hardware components such as general purpose centralprocessing units CPUs, such as those found in personal computers (PCs),embedded processors, graphic card processors, digital signal processors(DSPs), field programmable gate arrays (FPGAs), or others. To implementat least parts of the video encoding or decoding, instructions may beneeded, and those instructions may be stored and distributed using oneor more non-transitory computer readable media. Computer readable mediachoices include compact disc read-only memory (CD-ROM), digitalvideodisc read-only memory (DVD-ROM), memory stick, embedded ROM, orothers.

In the following, certain systems, methods and/or aspects relating in atleast one broad aspect to video compression and decompression, i.e., theoperations performed in a video encoder and/or decoder, will bedescribed. A video decoder may perform all, or a subset of, the inverseoperations of the encoding operations. Unless otherwise noted,techniques of video encoding described herein are intended also toencompass the inverse of the described video encoding techniques (namelyassociated video decoding).

The uncompressed, digital representation of video can be viewed as asample stream, wherein the samples can be processed by the video displayin scan order. One type of boundary often occurring in this samplestream is the boundary between pictures in the sample stream. Many videocompression standards recognize this boundary and often divide the codedbitstream at these boundaries, for example through the insertion of apicture header or other metadata at the beginning of each uncodedpicture.

For some applications, it may be advantageous to segment the codedpicture into smaller data blocks, which segmenting can occur prior to,or during, the encoding. Two use cases for which picture segmentationmay be advantageous are described below.

The first such use case involves parallel processing. Previously,standard definition video (e.g., 720×480 or 720×576 pixels) was thelargest format in widespread commercial use. More recently HD (up to1920×1080 pixels) formats as well as 4k (4096×2048 pixels), 8k(8192×4096 pixels), and still larger formats are emerging and findinguse in a variety of application spaces. Despite the increase inaffordable computing power over the years, as a result of the very largepicture sizes associated with some of these newer and larger formats, itis often advantageous to leverage the efficiency of parallel processingto the encoding and decoding processes. Parallel encoding and decodingmay occur at the instruction level (e.g., using SIMD), in a pipelinewhere several video coding units may be processed at different stagessimultaneously, or on a large structure basis where collections of videocoding sub units are processed by separate computing engines as separateentities (e.g., a multi-core general purpose processor). The last formof parallel processing requires picture segmentation.

The second such use case involves picture segmentation so as to create abitstream suitable for efficient transport over packet networks. Codecswhose coded video is transported over IP or other packet networkprotocols can be subject to a maximum transmission unit (“MTU”) sizeconstraint. It is sometimes advantageous for the coded slice size to besuch that the resulting packet containing the coded slice is as close tothe MTU size as possible without exceeding that size, so as to keep thepayload/packetization overhead ratio high, while avoiding fragmentation(and the resulting higher loss probability) by the network.

The MTU size differs widely from network to network. For example, theMTU size of many Internet connections may be set by the smallest MTUsize of network infrastructure commonly used on the Internet, whichoften corresponds to limitations in Ethernet and may be roughly 1500bytes.

The number of bits in a coded picture depends on many factors such asthe source picture's dimensions, the desired quality, the complexity ofthe content in terms of suitability for prediction, the codingefficiency of the video coding standard, and other factors. However,even at moderate quality settings and content complexity, for sequencesof HD resolution and above, the size of an average coded picture easilyexceeds the MTU size. For example, a video conferencing encoder canrequire about 2 Mbits/sec to encode a 720p60 video sequence. Thisresults in an average coded picture size of roughly 33000 bits or 4125bytes, which is considerably more than the approximately 1500 bytes ofthe Internet's MTU size. At higher resolutions, the average picture sizeincreases to values significantly above the Internet's MTU size.Assuming a similar compression ratio as in the 720p60 example above, a4096×2048 (4k) video at 60 fps (4kp60) may require over 300,000 bits, or25 MTU-sized packets for each coded video picture.

In many video coding standards, a picture segment (or, at least, oneform of a picture segment) is known as a “slice”. In the followingdescription, any kind of (e.g., video coding standard based) codedpicture fragmentation that breaks any form of in-picture prediction orother coding mechanism may be referred to generally as a “slice”. Assuch, structures such as the Group Of Blocks (“GOB”) in ITU.T Rec. H.261or ITU Rec. H.263 (available from the ITU; see above for H.264), slicesin H.264 or the MPEG family of standards, may each constitute a “slice”as this term is used herein throughout. However, fragmentation units ofRFC3984 or data partitions of 11.264 may not constitute a “slice”, asthis term is used herein throughout, even if they subdivide thebitstream of a coded picture into smaller datablocks, because they donot break in picture prediction or another coding mechanism.

One advantage of using slices over media unaware segmentationmechanisms, such as, for example, those provided by IP at the routinglayer, is that slices are at least to a certain extent independentlydecodable (as discussed below in more detail). The loss of one slicetherefore does not necessarily render the other slices of a codedpicture unusable or un-decodeable. Depending on the implementation of afragmentation mechanism, the loss of a fragment, in contrast, may wellrender many other fragments unusable.

Many or all in-picture prediction mechanisms or coding mechanisms maybroken by the decoding of a picture header (or equivalent). Whetherthose prediction mechanisms are broken also by the detection of a sliceheader may depend on the video compression standard, and the type ofslice in use.

In H.264, individual video pictures may be segmented into one or moreslices, thereby accommodating applications requiring or otherwiseutilized pictures that are partitioned as part of the encoding/decodingprocess. Slices in H.264 may be independently decodable with respect tomotion vector prediction, intra prediction, CA-VLC and CABAC states, andother aspects of the H.264 standard. While this decoding independencemay realize increases in error resilience, disallowing theaforementioned prediction across slice boundaries may tend to reducecoding efficiency.

In H.263, a video encoder has more flexibility in selecting whichprediction mechanisms are broken through the use of slices or GOBs withnon-empty GOB headers. For example, there is a bit included in thepicture header, selectable when Annex R is in use, which signals to thedecoder that no prediction at all occurs across slice/GOB boundaries. Ifthe bit is not set, though, motion vectors may point outside of thecurrent slice, thereby potentially “importing” sample values that areused for motion compensation within the current slice. Further, loopfiltering may incorporate sample values outside of the slice.

In most or all existing video coding standards, with the possibleexception of flexible macroblock ordering (“FMO”) used as part of H.264,macroblocks within slices are ordered in raster scan order.Consequently, when video sequences with large picture sizes arepartitioned into slices that encompass only a relatively smallpercentage of all macroblocks in the picture, the slices tend to beelongated when viewed spatially.

FIG. 1 shows an example picture 100 which is broken into slices inaccordance with the prior art. Example picture 100 has a matrix 101 of6×4 macroblocks, their boundaries indicated through hairlines. Thepicture 100 is divided into two slices 102, 103, with slice boundary 104between the two slices 102, 103 indicated by a bold line. The firstslice 102 contains 10 macroblocks in scan order, specifically,macroblock 1 through 10. The second slice 103 contains the remaining 14macroblocks in the matrix 101 (i.e., macroblocks 11 through 24). Thenumerals in the macroblocks (e.g., numeral ‘11’ in macroblock 105) arethe addresses of the macroblocks according to scan order.

The bitstream 106 represents the coded picture corresponding to picture100, and can include one or more parameter sets 107 as an example of ahigh level syntax structure, which can include syntax elements relevantto more than one of the coded slices of the picture 100. The parameterset(s) 107 can be followed by one or more slices, each such slicecomprising a corresponding slice header 108, 110, and correspondingslice data 109, 111, respectively. Accordingly, in this example, sliceheader 108 may be associated with slice data 109 and may correspond toslice 102 in matrix 101, while slice header 110 may be associated withslice data 111 and may corresponding to slice 103. The slice headers108, 110 may include information such as the address of the firstmacroblock of that respective slice, according to scan order. Forexample, the second slice 103 when coded into bitstream 106 starts withslice header 110 that includes a first macroblock address of ‘11’, whichdesignates the address of macroblock 105.

As can be seen in FIG. 1, slices 102 and 103 are somewhat elongated inthe sense that each of slices 102 and 103 span more macroblockshorizontally (i.e., 6 macroblocks) than vertically (i.e., 2 to 3macroblocks). Elongated slices such as slices 102 and 103 tend tocontain diverse picture content as a result of the large distance fromend to end horizontally. Further, elongated slices tend to have lowratios of slice area to slice perimeter/boundary. The combination ofslices containing diverse picture content with relatively low area toperimeter/boundary ratios can be disadvantageous from a codingefficiency perspective when compared with a slice that encompasses amore squared area of a picture, such as squares or other geometricfigures close to a square. Slices with this geometric property mayhenceforth be called “compact” slices within this description.

Also, many entropy coding tools that have two-dimensional properties,such as the coding of motion vectors or intra prediction modes, may beoptimized for squared picture aspect ratios. For example, in H.264, thecoding of a horizontal motion vector of a given length costs roughly thesame number of bits as the coding of a vertical motion vector of thesame length. Consequently, these coding tools may yield a bettercompression for compact slices than for “elongated” slices, such asslices 102 and 103 shown in FIG. 1.

Improved coding efficiency for compact slices may further arise from thefact homogenous content, which is more likely to be found in a compactslice, may be more efficiently encoded as compared with the relativelydiverse content that is more likely to be found in an elongated slice.As a general though not necessarily absolute rule, picture content ismore likely to be homogenous in a compact slice because the spatialdistance from the center to the boundaries of the slice is less, onaverage, for a compact slice than for an elongated slice. Further,having a higher slice area to slice boundary ratio for compact slicesmeans that fewer prediction mechanisms may generally be broken in agiven picture, thereby resulting in higher coding efficiency.

In H.264, FMO allows the video encoder to effectively producerectangular slices by defining rectangular slice groups. FMO is a highlygeneralized coding tool that was designed to address several issuesencountered in video coding. However, from a practical standpoint, FMOtends to be perceived as having a relatively high degree ofimplementation complexity, resulting in somewhat limited adoption as anaspect of standard video compression. A simpler coding tool that mayrealize improved coding efficiency, as well as parallel encoding anddecoding, may address or ameliorate one or more of the complexity issuesassociated with a full FMO implementation.

The issue of elongated slices may also appear in an extreme case in manyMPEG-2 based encoding schemes. For example, in MPEG-2 encoding, it isoften the case that each single row of macroblocks within a picture isencoded into a slice, thereby effectively breaking any in pictureprediction mechanisms in the vertical dimension within the picture.

Rectangular slice mode is one of two sub-modes specified in Annex K ofH.263, another being “scan order slice mode”, which has propertiessimilar to the slices of 11.264 discussed above. Rectangular slices asprovided for in H.263 may offer one or more of the earlier describedadvantages that compact slices provide. However, H.263 requires that thedimensions (specifically the width) of each slice must be conveyed inits corresponding header, which leads to coding inefficiency, forexample, in applications in which the slice sizes in the horizontaldimension do not change from picture to picture. In addition, Annex K ofH.263 does not specify a minimum slice width that would effectivelyprevent vertically elongated slices from being used. Verticallyelongated slices may introduce implementation difficulties and would notin every case provide the desired coding efficiency advantages that, forthe reasons discussed above for horizontally elongated slices, may beprovided through use of more compact slices.

Constraining the slice to have a rectangular shape can also bedisadvantageous in certain cases. First, rectangular slices may performsub-optimally in applications for which the bitstreams use transportprotocols subject to an MTU. For example, packets may be fragmented ifthe number of bits within a give packet exceeds the MTU limit imposed onthe bitstream, which can be undesirable from at least networkperformance and error resilience perspectives. Conversely, if the numberof bits within a given packet is far below the MTU limit, then the ratioof the number of bits in the transport and slice headers becomesrelatively large as compared with the number of bits in the packetpayload, thereby leading to coding inefficiencies. Requiring slices tobe rectangular in shape limits the encoder's ability to preciselycontrol the number of bits in the coded slice so as to avoid theabove-mentioned disadvantages.

Second, rectangular slices may perform sub-optimally in applicationsthat utilize parallel encoding and/or decoding. When encoding and/ordecoding in parallel, it is typically advantageous to partition apicture into different parts such that each part of the picture requiresapproximately the same amount of computational power to encode. Bypartitioning the picture in this way, each part of the picture maytherefore be encoded with nearly the same latency to thereby reduce orminimize lag between the encoding times of different parts of thepicture. An encoder constrained to use rectangular slices may not beable to precisely control the amount of CPU capacity required to encodeand/or decode each slice and thereby avoid this potential disadvantage.

In order to facilitate parallel decoding of slices belonging to the samecoded picture, a decoder will generally assign coded picture segments tothe various processors, processor cores, or other independentlyoperating decoding mechanisms made available to the decoder for paralleldecoding. Without the use of FMO, this was a generally difficult, insome cases extremely difficult, task for previous video coding standardsto handle, as those previous standards would allow too much flexibilityin the bit stream generation. For example, in H.264, it is possible thatone picture may be coded in a single slice and another picture intodozens of slices within the same bitstream. If parallelization occurs atthe slice level, when a picture is coded in a single slice, theprocessor assigned to decode that picture will need to be provisioned tohandle its decoding in full. As a result, without imposing restrictionsoutside of the video coding standard, there may be comparatively littleadvantage realized by building parallel decoders if each decodingprocessor will need to be provisioned to be capable of handling a wholepicture in any event.

The slice coding used in many MPEG-2 encoders is widely viewed to be theresult of an agreement to utilize an informal Cable Labs specificationthat suggested a one slice per macroblock row segmentation scheme.Widespread acceptance of this informal specification was eventuallygained. While there may have been value in such a segmentation schemewhen the first MPEG-2 products became available, around 1995, today thevarious restrictions associated with the historical specification maysignificantly limit coding efficiency, although parallelization ofdecoding of (at least SD-coded) pictures has been a relative non-issuefor at least a decade.

A need therefore exists for an improved method and system for picturesegmentation that addresses, ameliorates or otherwise provides a usefulalternative to the existing shortcomings of video encoders both in termsof MTU size matching and parallel decoding. Accordingly, a solution thataddresses, at least in part, the above and other shortcomings isdesired.

SUMMARY

Embodiments of the invention relate in one or more broad aspects totechniques for segmenting a coded picture into columns and slices.

In some embodiments, a plurality of columns may be defined in one ormore syntax elements placed in one or more high level syntax structures,such as slice headers or parameter sets. The columns so defined may beof any width between, for example, one Coded Tree Block (CTB, as definedin, for example in WD4: (B. Bross et. al., “WD4: Working Draft 4 ofHigh-Efficiency Video Coding”, available fromhttp://wftp3.itu.int/av-arch/jctvc-site/2011_07_F_Torino/) and theentire horizontal size of the picture as measured in CTBs.

In some embodiments, a column boundary between adjacent columns maybreak one or more prediction mechanism or loop filtering mechanisms ofthe video codec. The prediction or loop filter mechanisms broken bycolumn boundaries may be the same or different from those broken byslice boundaries.

In some embodiments, the granularity for calculating and specifyingcolumn width can be a Largest Coding Unit (LCU), and a coded tree block(CTB) may be of the same size as an LCU.

In some embodiments, N columns may be defined by a syntax element N thatindicates the number of columns in a picture, and which may be followedby N−1 syntax elements that indicate the corresponding widths of each ofthe N columns. The width of a designated column, for example theright-most column, may be computed, for example, by subtracting the sumof the widths of the left-most N−1 columns from the width of thepicture.

In some embodiments, each column in the picture may have an equal width(except perhaps for one pre-defined column, such as the rightmostcolumn, which can be of lesser width) and a syntax element W may be usedto indicate such equal width of every column (except the one pre-definedcolumn. The syntax element W may be placed, for example, in a high levelsyntax element structure such as a parameter set. In such cases, thewidth of the one pre-defined column can be determined based on adivision operation involving W and the width of the picture, representedby N, whereby the width of the one pre-defined column may be taken asthe remainder when N is divided by W.

In some embodiments, each column in the picture may have an equal width(except perhaps for one pre-defined column, such as the rightmostcolumn, which can be of lesser width, in case the width of a picture inCTBs is not an integer divisible of N.), and a syntax element N may beused to indicate the number of equally wide columns (exclusive perhapsof the one pre-defined column). In such cases, the width of all Ncolumns may be determined by dividing the width of the picture in CTBsby N, with the width of the pre-defined column then equaling theremainder of the division operation.

In some embodiments, a syntax element N may be used to indicate thenumber of columns in a picture. The equal column width W, in units ofLCUs, equals the width of the picture, in units of LCUs, divided by N.

In some embodiments, if the picture width is not an integer divisible bythe equal column width W, then one pre-defined column, for example therightmost column, may have a width that is the remainder of a divisionoperation between the width of the picture (measured, for example, inCTBs) and W−1.

In some embodiments, a slice may encompass one or more CTBs each locatedwithin a single column, thereby enabling MTU size matching, while stillallowing for parallelization through columns.

In some embodiments, a slice may encompass CTBs that are located in morethan one column.

In some embodiments, the width of a column may be adjusted in a sliceheader or comparable syntax structure, thereby allowing to “loadbalance” the computational resources in the encoder. Such adjustment ofcolumn width may be advantageous if the content complexity is differentin different horizontal areas in a source picture sequence.

In some embodiments, a high level syntax element may be used to fix thesize of all columns of a picture, thereby allowing an efficientassignment of each column to a single processor or equivalent in aparallel encoding or decoding scenario.

In some embodiments, a constraint for the minimum and/or maximumhorizontal width of a column may be imposed. Such constraint may in somecases aid parallelization and can improve coding efficiency. Theconstraint may, for example, be codified in the level specification of astandard, or in a high level syntax element, or can be adapted byoptimizing a function that can involve MTU size and average bits per CTB(which can be derived from bitrate, frame rate, and frame size in CTBs).

In some embodiments, a scan order of CTBs may be used for decodingand/or encoding, according to which CTBs may be processed in an orderedsequence, at a first level, by column from left-to-right across allcolumns in the picture, and a second level below the first level, by CTBfrom left-to-right and top-to-bottom across all CTBs within a givencolumn.

In one broad aspect, there is provided a method for encoding a videopicture comprising a plurality of coded tree blocks (CTBs) arranged intoat least two rows and at least one column of CTBs. The method mayinvolve encoding a corresponding column width of each at least onecolumn, each column width greater than zero and a sum of all columnwidths equal to a horizontal dimension of the video picture, andencoding at least one slice of the video picture. In some cases, the atleast one the slice may include at least two CTBs located in at leasttwo rows of CTBs.

In another broad aspect, there is provided one or more non-transitorycomputer-readable media on which are stored instructions arranged toenable one or more processors to perform a method for video encoding avideo picture comprising a plurality of coded tree blocks (CTBs)arranged into at least two rows and at least one column of CTBs. Themethod may include encoding a corresponding column width of each atleast one column, each column width greater than zero and a sum of allcolumn widths equal to a horizontal dimension of the video picture, andencoding at least one slice of the video picture. In some cases, the atleast one slice may include at least two CTBs located in at least tworows of CTBs.

In some embodiments, according to either of the above two aspects, themethod may further include breaking at least one form of prediction orin-loop filtering at column boundaries between adjacent columns of CTBs.

In some embodiments, according to either of the above two aspects, themethod may further include encoding at least one of a number of columns,and a width of a column into a parameter set.

In some embodiments, according to either of the above two aspects, thecorresponding column width of at least one column may be variablebetween at least two rows of CTBs in the video picture. In suchembodiments, such variability may be controlled by coding thecorresponding column width in a slice header.

In some embodiments, according to either of the above two aspects, themethod may further include encoding the plurality of CTBs following ascan order of CTBs, according to which the plurality of CTBs are encodedsequentially by column from left-to-right across a plurality of columns,and sequentially by CTB from left-to-right and top-to-bottom within eachof the plurality of columns.

In yet another broad aspect, there is provided a method for decoding acoded video picture comprising a plurality of coded tree blocks (CTBs)arranged into at least two rows and at least one column of CTBs. Themethod may include obtaining at least one of (i) a corresponding columnwidth of each at least one column and (ii) a number of columns, derivingthe corresponding column width of each at least one column (if notobtained from the coded video picture), and decoding at least one sliceof the coded video picture. In some cases, the at least one slice mayinclude at least two CTBs located in at least two rows of CTBs.

In yet another broad aspect, there is provided one or morenon-transitory computer-readable media on which are stored instructionsarranged to enable one or more processors to perform a method for videodecoding a coded video picture comprising a plurality of coded treeblocks (CTBs) arranged into at least two rows and at least one column ofCTBs. The method may include obtaining at least one of (i) acorresponding column width of each at least one column and (ii) a numberof columns, deriving the corresponding column width of each at least onecolumn (if not obtained from the coded video picture), and decoding atleast one slice of the coded video picture. In some cases, the at leastone slice may include at least two CTBs located in at least two rows ofCTBs.

In some embodiments, according to either of the above two aspects, themethod may further include breaking at least one form of prediction orin-loop filtering at column boundaries between adjacent columns of CTBs.

In some embodiments, according to either of the above two aspects, atleast one of the corresponding column width of the at least one columnand the number of columns may be obtained from a parameter set.

In some embodiments, according to either of the above two aspects, thecorresponding column width of at least one column may be variablebetween at least two rows of CTBs in the coded video picture. In suchembodiments, such variability may be controlled by coding a column widthin a slice header.

In some embodiments, according to either of the above two aspects, themethod may further include decoding the plurality of CTBS following ascan order of CTBs, according to which the plurality of CTBs are decodedsequentially by column from left-to-right across a plurality of columns,and sequentially by CTB from left-to-right and top-to-bottom within eachof the plurality of columns.

In yet another broad aspect, there is provided a video decoder for acoded video picture comprising a plurality of coded tree blocks (CTBs)arranged into at least two rows and at least one column of CTBs. Thevideo decoder may include a plurality of column decoders, with eachcolumn decoder configured to decode slices of the coded video picturelocated within at least one column of CTBs allocated to that columndecoder.

In some embodiments, according to the above aspect, each slice may befully located within exactly one column.

In some embodiments, according to the above aspect, each column decodermay be a process running on at least one of a core of a multicoreprocessor, or a multiprocessor.

In accordance with further aspects of the described embodiments, thereis provided an apparatus such as a data processing system, a method foradapting such apparatus, as well as articles of manufacture such as anon-transitory computer readable medium or product having programinstructions recorded thereon for practising the described method(s).

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the embodiments of the presentinvention will become apparent from the following detailed description,taken in combination with the appended drawings, in which:

FIG. 1 is a diagram illustrating an exemplary picture having scan orderslices and macroblocks addresses;

FIG. 2 is a diagram illustrating columns and slices in accordance withan embodiment of the invention;

FIG. 3 is a diagram illustrating columns and slices (local to a givencolumn) in accordance with an embodiment of the invention;

FIG. 4 is a flow diagram illustrating the operation of an exemplarydecoder in accordance with an embodiment of the invention;

FIG. 5 is a diagram illustrating the modification of column width inaccordance with an embodiment of the invention;

FIG. 6 is a flow diagram illustrating the operation of an exemplarydecoder when decoding a slice in accordance with an embodiment of theinvention;

FIG. 7 is a block diagram of a decoder with two column processors inaccordance with an embodiment of the invention; and,

FIG. 8 is a block diagram illustrating a data processing system (e.g., apersonal computer (“PC”)) based implementation in accordance with anembodiment of the invention.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, details are set forth to provide anunderstanding of the invention. In some instances, certain software,circuits, structures and methods have not been described or shown indetail in order not to obscure the invention. The term “data processingsystem” is used herein to refer to any machine for processing data,including the computer systems, wireless devices, and networkarrangements described herein. Embodiments of the present invention maybe implemented in any computer programming language provided that theoperating system of the data processing system provides the facilitiesthat may support the requirements of these embodiments. Embodiments ofthe present invention may also be implemented in hardware or in acombination of hardware and software.

Embodiments of the present invention relate to picture segmentation invideo compression using columns and slices.

Referring to FIG. 2, there is shown an exemplary segmentation 200 of apicture 201 in accordance with embodiments of the invention. The picture201 is divided into a plurality of Coded Tree Blocks or macroblocks(henceforth referred to as “CTBs”). In this example, the picture 201 hasdimensions of 6×4 CTBs. The boundaries of CTBs are shown generally ashairlines. The nature of the CTBs, and their specific dimensions, isvariable and may be altered according to different embodiments. Whilethe description below assumes that that CTBs included within picture 201are square and of equal size (as it is common for macroblocks in H.264,for example), certain embodiments of the invention may not require sucha property and can may be operable with CTBs of differing size(s),provided that in such embodiments it is possible to define one or morehorizontal columns using those unevenly sized CTBs. For example,embodiments of the invention may be operable with macroblock pairs asused in H.264 interlace coding, or with the reduced resolution updatemode of H.263. The embodiments may also be operable if there were CTBsof, for example, 32×32, 16×16, and 8×8 samples in the same picture.

Two columns 202, 203 are defined in the picture 201 and are showndivided by a column boundary depicted as boldface, solid line 204. Thewidth of any column can be an integer multiple of the size of a CTB (oran integer multiple of the size of the smallest CTB possible) usedwithin the picture 201. Columns may be of equal or unequal width withinthe picture 201. As shown, column 202 encompasses CTBs 1 through 8, andcolumn 203 encompasses CTBs 9 through 24. Column 202 is two CTBs wide,whereas column 203 is four CTBs wide. However, the described embodimentsare not limited to this number of columns nor to the exemplary columnwidths shown in FIG. 1.

Shown also are two slices defined within picture 201 and delineated bythe boldface punctuated bordering line 205. The first slice encompassesthe CTBs with CTB addresses 1 through 14. The second slice encompassesCTBs with addresses 15 through 24. CTBs 1-8 of the first slice arelocated in column 202, whereas CTBs 9-14 of the first slice are locatedin column 203. Therefore, the first slice covers CTBs that are part ofeither column 202 and 203. The second slice includes CTBs 15-24, whichare all located in column 203. Accordingly, the scan order of CTBswithin picture 201 has changed from the per-picture scan order, as shownin FIG. 1, to a per-column scan order, as shown in FIG. 2. In accordancewith the per-column scan order, the CTBs of column 202 are firstenumerated in a column-local scan order (left-to-right, top-to-bottomthroughout column 202), before the CTBs of column 203 are enumerated (bya column-local scan order defined in a similar way). If/whereappropriate, the per-column scan order shown in FIG. 2 can be extendedto pictures for which more than two columns have been defined.

The coded bitstream 206 represents the picture 201 and can contain aparameter set 207 (or can include a reference to a parameter set thathas previously been decoded, as multiple pictures can refer to the sameparameter set). The parameter set 207 can contain column boundaryinformation 208, identifying directly or indirectly the width of one ormore column(s), e.g., columns 202 and 203, as described later. Each ofthe coded slices can comprise a corresponding slice header 209, 211, andcorresponding slice data 210, 212, respectively.

Picture segmentation with slices spanning column boundaries, as shown inFIG. 2, can provided one or more advantages, such as that the size ofall coded slices (with the possible exception of the last slice of agiven picture) can be adjusted to match the MTU size 213 as closely aspossible, at least partly on account that coded slices may not includeparts of coded CTBs. In the example of FIG. 2, the two coded slices(each comprising respective slice headers 209, 211 and slice data 210,212) are shown as roughly of the same size so as to indicate that thesecoded slices carry roughly the same number of bits, which can be anumber smaller, but close to, the MTU size 213. The MTU size 213 isshown as an interval, which, in this example, is constant for the twoslices. While, in most cases, the MTU size 213 is a network property andconstant for any packet, for the purpose of optimizing packet payloadthrough the use of slices, other consideration(s) may urge adoption of avariable target size for coded slices. For example, for errorresilience, it may be useful for some packets to carry redundant copiesof parameter sets in addition to their slice payload. In such cases, theMTU size 213 may differ from packet to packet and, therefore, from codedslice to coded slice as well. The term “MTU size” is used in this broadsense throughout the description.

One advantage of allowing slices to span multiple columns across theboundaries between them is to allow for good MTU size matching. Withslices able to span multiple columns, none of the slices will beartificially limited in size due to the presence of a column boundary.Another advantage can be that it is possible to represent the wholepicture as a single slice (for example, where the MTU size is, in mostcases, larger than the coded picture size), while still allowing forparallelization. For convenience, it may be assumed that a columnboundary breaks all forms of prediction or in-loop filtering. Under thisassumption, one example configuration of a parallel encoder may be asfollows. Each processor/core of the parallel decoder may begin encodingthe spatial area of a column, starting with the top-left CTB within thecolumn and working sequentially in scan order to the bottom rightmacroblock, i.e., left-to-right and top-to-bottom. Assuming N columns ina picture and N processors/cores with the parallel decoder, thisapproach to encoding may results in N sub-bitstreams being generated,one from each processor/core. The N sub-bitstreams may then be combinedto form a single bitstream representing the whole picture. A decoderreceiving such bitstream may, from the parameter set (e.g., included inthe bitstream), ascertain that it is being tasked for parallel decodingin N columns. The decoder may pre-process the combined bitstream byfirst decoding only syntax elements included in the bitstream (withoutalso performing any computationally intensive reconstruction of CTBs) soas to identify the boundaries between the columns in the bitstreamdomain. As an example, one such boundary would be located between codedCTB 8 and coded CTB 9 of picture 201. Thereafter, the decoder mayextract and/or copy the sub-bitstreams, broken up within the combinedbitstream at the pre-identified boundaries, to the processors/cores forprocessing. The processors/cores within the decoder may independentlyreconstruct their assigned columns in the pixel domain, which may thenbe copied together to reconstruct the full image. Other possibleimplementation strategies are explained in more detail below.

One potential disadvantage of having slices spanning across columnboundaries can arise because of difficulties in the implementation ofparallelization, for example, when only a (small) subset of predictionand/or loop filtering mechanisms are broken by a column boundary. Forexample, if column 202 were to be decoded on a first processor, andcolumn 203 were to be decoded on a second processor, then the state ofthe decoding engine (including information such as CABAC state, motionprediction state, and so on), or at least those parts of it which relateto prediction or loop filter tools not broken by column boundaries,would in at least in some cases have to be handed over from the firstprocessor to the second processor after the decoding of CTB 8, which incolumn-local scan order is the last macroblock in column 202. Theimplementation and computational complexity of this handover can dependon, for example, the prediction and loop filter tools that are disruptedby the presence of a column boundary. If, for example, a column boundarybreaks any form of prediction but the prediction in the entropy coding,then only information pertaining to the entropy coding engine needs tobe handed over. Co-pending U.S. patent application Ser. No. 13/336,475,filed even date herewith, entitled “METHOD AND SYSTEM FOR SELECTIVELYBREAKING PREDICTION IN VIDEO CODING”, which is incorporated herein inits entirety, discloses, among other things, techniques to control theamount of data that needs to be handed over when a slice spans a columnboundary.

FIG. 3 illustrates the interaction between slices and columns of a videopicture in a less general case 300, in particular, which is optimizedfor parallel processing of equally powerful processors (or otherparallel processing entities such as processor cores), at the expense ofthe best possible MTU size matching. (Such optimization may also dependon the prediction and loop filtering tools that are broken through theuse of columns; co-pending U.S. patent application Ser. No. 13/336,475,discloses, among other things, techniques to control the tools affectedby column boundaries.)

As seen in FIG. 3, a picture 301 is divided in two columns 302, 303 ofequal width (in this example, three CTBs wide), resulting in a similarnumber of CTBs to be assigned to each of columns 302, 303. Under theassumption that a given column is processed by a given processor orcore, division of picture 301 into columns 302, 303 of equal widths alsomay result in roughly equal load for each processor or core. The columnboundary 304 between the columns 302 and 303 is shown as a boldfaceline. All CTBs of column 302 are coded in a single slice 305. Whereas incolumn 303, two slices 306, 307 are defined and collectively include allof the CTBs of column 303, with each slice 306, 307 defined so as tostay within MTU size constraints. For example, slices 305, 306 and 307may each satisfy MTU size constraints, despite containing a differentnumber of macroblocks (twelve, eight, and four, respectively) where ahigher average number of bits is required for the CTBs of column 303relative to the CTBs of column 302, which can occur if/when one part ofthe picture 201 is more “complex” than the other. The slice boundary 308between slices 306 and 307 within column 303 is indicated by a boldface,punctuated line. It should be noted that slices 305 and 306 areconsiderably more “compact” than would have been slices containing asimilar number of CTBs but arranged in picture scan order. Therefore,the use of columns 302 and 303 may not only facilitate the use ofparallel processing (as will be described in greater detail below), butalso can aid in coding efficiency. Slice 307, in this example, while notbeing very compact as compared to slices 305 and 306 may still be morecompact than it would have been without the use of columns 302 and 303.Once more, the scan order of CTBs is changed as previously described.

The coded bitstream 309 can contain a parameter set 310 comprisingcolumn boundary information 311. While the parameter set 309 may beshown in FIG. 3 to be adjacent to the coded slices of the coded picture,in various embodiments, it may be possible for the parameter set to belocated elsewhere in the bitstream 309 or even in some cases to beconveyed out of band.

The coded picture contains three coded slices (i.e., slice 0, slice 1and slice 2), each comprising a corresponding slice header 312, 314,316, and corresponding slice data 313, 315, 317, respectively. The codedslice 0 includes coded slice header 312 and coded slice data 313, whichincludes the coded information of slice 305, and is depicted having asize (in coded bits) somewhere below the MTU size 318. Coded slice 1includes coded slice header 314 and coded slice data 315 (correspondingto slice 306). In this example, when coding slice 1, the encoder is ableto fill the packet almost up to MTU size 318 with slice header 314 andslice data 315. Note that it may not always be possible to fill thepacket up to the last bit of the MTU size 318 because it is not possibleto fragment a CTB into more than one slice. Accordingly, subject to thisrestriction, there may be extra bits leftover after encoding of the lastCTB within the slice. Finally, slice 2 includes slice header 316 andslice data 317 (corresponding to slice 307), and is considerably smallerthan the MTU size 318 because there are fewer CTBs to be coded in thisslice, e.g., as compared to slice 0 and slice 1.

A static width of the columns within a picture can be specified at thesequence level (e.g., in a sequence parameter set), at the picture level(e.g., in a picture parameter set), or in other data structures in thebitstream covering at least one coded picture. Specifying columnboundaries once for all slices in a sequence or picture has advantagesfor applications whose slices do not vary in width from picture topicture, in that the overhead associated with specifying a differentslice structure is not required for every picture. It further has theadvantage of efficiently allowing the distribution of coded bits amongmultiple processors or equivalent in a parallel decoding scenario.Dynamic column width adjustments are described later.

Column boundaries may be specified using one or more different datastructures, parameters, rules, flags, or other suitable syntax. Usingthe sequence parameter set (SPS) as the exemplary high level syntaxstructure to convey column boundaries, the SPS can for example include:

(1) a variable N by which is coded the number of columns per picture. Insuch cases, if/when necessary and/or appropriate, the width of eachcolumn (in units of CTBs) may be derived by dividing the width of apicture (in units of CTBs) by N, if the width of the picture isdivisible by N. Otherwise, the width of N−1 columns may be calculated bydividing the width of the picture by N−1, whereby the width of the finalcolumn (for example the rightmost column in the picture) would then bethe remainder of the division operation.

(2) a variable W by which is coded the maximum width (in units of CTBs)of a column. In such cases, let N be width of the picture divided by W.Then the width of the first N columns is equal to W, and the width ofthe final column would be the remainder of the division operation.

(3) a variable N containing the number of columns, and a field ofintegers N−1 long containing values to represent the widths of each suchcolumn. This option allows for non-uniform column width as shown in FIG.2.

Still other representations of column boundary information which aresuitable for different applications may be apparent afterfamiliarization with the present disclosure and are intended to becovered within the scope of the described embodiments.

A video coding standard can advantageously set limits for minimum ormaximum column width that can occur in a bitstream. The limits can bespecified, for example, in profile and level specifications. Forexample, a profile designed for high level parallel processing in caseof high resolution video decoding, can require that columns must be usedwith a width no larger than, for example, 1920 samples (a 1080presolution, which is commonly decodable in a single core, has aresolution of 1920 horizontal samples). On the other hand, the profilecan require, for example, a minimum column width of no less than 640samples. The lower limit can help to avoid worst-case scenarios and“elongated” slices in the Y dimension. The upper limit can be useful fora number of different reasons. First, as already described, codingefficiency can be improved due to fewer and less elongated slices.Second, the decoder can be based on sub-picture decoders each capable ofdecoding a single column (henceforth called “column decoders”), but notnecessarily capable of handling a full picture. Third, memoryrequirements in the column decoders can be reduced as line buffers donot need to span the full picture width, but only the column width (plusperhaps implementation dependent padding). Other advantages of such anupper limit on column width may also be apparent.

Described next is the operation of a decoder on a bitstream containingslices and columns, noting how such operation may differ from thedecoding process of a bitstream that has been generated without use ofcolumns. For convenience, in the following description it will beassumed that coding of column width was performed according to option(3) above. However, the description should not be understood as beinglimited only to this type of coding and will apply also to the other twooptions described above, if/where necessary, by making suitablemodification and/or alteration to the description.

In modern video coding standards, parameters pertaining to more than oneslice can be stored in higher level syntax structures known as parametersets. In older standards, these parameters can be stored in headers suchas the picture, GOP, or sequence headers. To facilitate the followingdescription, parameter sets are assumed henceforth, although embodimentsof the invention may equally utilize other forms of high level syntaxstructures.

FIG. 4 shows a method 400 that illustrates the operation of an exemplarydecoder in accordance with embodiments of the invention. First, so as tobe able to decode slice data, one or more parameter sets may be decoded(401) as parameters stored in the parameter sets and which may beaccessed through a reference stored in the slice header. In someembodiments, the parameter sets can contain information pertaining to,and which is sufficient to completely define, the column mechanism usedwithin the pictures to be decoded. For example, the parameter sets mayinclude parameters used to indicate the number of columns (i.e.,“num_column_minus_1” in FIG. 4), and the width of each such columnexclusive of the last column (i.e., “columnWidth[ ]” in FIG. 4). Thewidth of the last column in the coded picture can equal the distancebetween the rightmost column boundary and the right hand pictureboundary.

The stored parameters (e.g., “num_column_minus_1” and “columnWidth[ ]”)may be decoded (402) as one part of the parameter set decoding (401).The decoding (402) may be a sub-step of the decoding (401) and may beperformed at any appropriate time during the parameter set decoding(401). The exact order may, for example, be constrained by the syntax ofthe parameter sets themselves (which can mandate the location of theinformation decoded in 402 relative to other entries in the parameterset).

During the parameter set decoding (401), information relating to theprediction and/or in-loop filtering tools can also be decoded, asdescribed further in co-pending U.S. patent application Ser. No.13/336,475. Decoding of information relating to prediction and/orin-loop filtering tools is indicated generally in FIG. 4 at 403, and mayalso be considered to be one part of the parameter set decoding (401).

Following the decoding (401) of parameter set information in method 400,slice decoding and reconstruction can commence.

First, a slice header can be decoded (404). In the slice header,information can be available that allows for the decoder to associatethe given slice with the column in which that given slice is included.For example, the slice header can contain a CTB address (i.e., asindicated by “first_ctb_address” in FIG. 4) that represents the addressof the first CTB included within the slice. The encoded CTB address maybe decoded (405) and used as the initial value for a parameter used bythe decoder to keep track of the current CTB address (i.e., “currentCTB”in FIG. 4) during the decoding process. Then, the column in which theslice resides can be identified (406) based on the address of the firstCTB included within the slice by associating that first CTB with aparticular column defined in the coded picture.

For example, referring back to FIG. 1, CTB addresses (of which the CTBaddress decoded at 405 is one instance) may be entropy codedrepresentations of the locations of each given CTB in the picture, inscan order, as indicated by numerals of FIG. 1 centred in the CTBs. Toassociate slices with columns, the horizontal component of the CTBaddress of the first CTB in the given slice can be compared to thecolumn boundaries, defined for the coded picture, which may be derivedfrom column width information obtained during the decoding of theparameter sets (and may possibly be modified during later slicedecoding, as outlined below). In some embodiments of the invention,where each given slice may be constrained to resides in its entiretywithin a given column, slices may be associated with columns byidentifying the column in which the first CTB within that given sliceresides (because, by definition, all remaining CTBs in the slice wouldbe located within the same column). This identification can beundertaken according to block 611 and 612 of FIG. 6, which are discussedin more detail below.

Referring again to FIG. 4, the first (initial) column for the slice tobe decoded is identified (406). The decoding of coded CTBs in the slicemay then proceed as specified in one or more video compression standardsfor the decoding of CTBs to which the decoding may comply, which caninclude steps such as entropy decoding, motion compensation to form apredictor, inverse quantization and inverse transform of coefficients tocalculate a residual, adding the residual to the motion compensatedpredictor, in-CTB deblock-filtering the resulting samples, and so forth,with the exception of the handling of CTB addresses and the use of dataoutside of the slice based on its availability for prediction. In thefollowing, one example outlining one possible implementation strategy isoutlined, although other strategies may be apparent as well afterfamiliarization with the present disclosure.

First, the CTBs texture, motion, and other information can be decodedand the current CTB being handled by the decoder can be reconstructed(407), which can involve any or all of the different steps noted abovein the context of CTB decoding, while also observing the availability ofprediction information outside of the slice or column boundary.

Then, the current CTB address may be incremented (408). If theincremented CTB address is determined (409) to still be left of thecolumn's right-hand boundary, no further updating of the CTB address isrequired (indicating in FIG. 4 by the arrow bypassing block 410 directlyto block 411). However, if it is determined (409) that the incrementedCTB address is outside the column (that is, equal or right of thecolumn's right-hand boundary), then the CTB address is reset (410) tothe address of the first CTB in the next line of CTBs within the column.In other words, the horizontal component of the CTB address is set tothe leftmost CTB of the column (as indicated by the parameter “colBound[]” in FIG. 6), and the vertical component of the CTB address isincreased by one.

If, after execution of blocks 408 to 410, the vertical component of theCTB address exceeds the vertical size of the picture, then the slice mayspan more than one column. In this case, the CTB address may be reset,for example, to the top-left CTB address in the column to the left ofthe previous column. Decoding and CTB address changes continue in thiscolumn.

At this point, it is determined (411) whether more data (indicating oneor more CTBs) is available in the slice to be decoded. If it isdetermined (411) that more data remains in the slice to be decoded, thenmethod 400 proceeds (412) back to block 407, and the decoding continueswith the next CTB within the slice. However, if it is determined (411)that no data remains in the slice to be decoded, then method 400proceeds (413) back to block 404, and the decoding continues with thenext slice in the picture.

According to embodiments of the invention, a decoder can read (initial)column boundary information from high level structures such as pictureor sequence parameter sets. For example, this can be implemented in amechanism that parses first a syntax element (i.e.,“num_column_minus_1”) from the bitstream. This syntax element mayadvantageously be part of a picture (or higher) syntax structure, suchas a picture parameter set, sequence parameter set, picture header, orcomparable structure. If the value of “num_column_minus_1” is greaterthan 0, then it may be inferred that the number of column boundariespresent in a conformant bitstream will be equal to the value stored inthis parameter, and there num_column_minus_1 column widths may beparsed. The widths of the correspondingly parsed columns can beindicated in terms of CTBs. The syntax described above is specified inthe style of the H.264 syntax specification, parts of which aresummarized below for convenience.

Specifically, in the syntax description provided below, a boldfacevariable (i.e., lines 1 and 4 below), in comparison to a plain formattedline, may represent bits that are fetched from the bitstream as a sideeffect of or otherwise in conjunction with determining the value of thevariable. Once read, the value can be used in the decoding processhowever the variable is no longer set in boldface, since the use of thevariable is from that point onwards no longer associated with abitstream fetch.

num _(—) column _(—) minus _(—) 1 if(num_column_minus_1 > 0) { for (i=0;i < num_column_minus_1; i++) { column Width[i] } }

If column boundaries can be specified at more than one level, accordingto the given compression standard, then the syntax of column boundariescan be the same at various levels, for example, sequence and/or picturelevel. In this case, advantageously, column boundaries indicated at alower relative level (i.e., the picture level) can take precedence overcolumn boundaries indicated at a higher relative level (i.e., thesequence level).

The parsed columnWidth[ ] values may be used to derive the columnboundaries as shown below:

if(num_column_minus_1 > 0) { columnBoundary[1] = columnWidth[0]; for(i=1; i < num_column_minus_1; i++) {columnBoundary[i+1]=columnBoundary[i] + columnWidth[i]; } }

In the following, embodiments of the present invention in which thecolumn boundaries can be adjusted during the coding of a picture, i.e.,at the slice level are described.

Referring to FIG. 5, there is shown a diagram illustrating themodification 500 of column width in accordance with embodiments of theinvention. In FIG. 5, the location of column boundaries can be modifiedat the slice level (e.g., based on information from a slice header). Themodification 500 may pertain only to the current picture and not toother pictures in the coded bitstream. When encoding processingparallelization is involved and columns may be processed by independentprocessors or similar units, the modification 500 may provide anadvantage in that the number of CTBs per column can be adjusted toprocessor load possibly from outside load factors (other processes notrelated to the video encoding), content complexity (that may vary fromcolumn to column), or still other factors.

In the following description, for convenience, the slice header is usedfor conveying modification information because, in certain contexts, theslice header may be the most appropriate syntax structure in H.264 forsuch purpose. The use of the slice header for information pertaining tomore than one slice is not necessarily in line with H.264's syntax model(for example, in H.264, all information that pertains to more than oneslice should be included in parameter sets only), but is in line withsome older video compression standards (for example, in H.261, GOBheader information can be predicted from previous GOB headers, and GOBsare at least in some respects similar to equivalent to H.264 slices). Atthe same time, if a mechanism were to be included in newer videocompression standards, which would allow the coding of properties at theslice level that pertains to more than just the current slice, then suchsyntax structure could in some cases be a more suitable location for theinformation presented below than in the slice header itself. Forexample, included in WD4 is an Adaptation Parameter Set (APS), which isdesigned to carry information that is likely to change from picture topicture (in contrast to the picture parameter set, which carriesinformation that is unlikely to change from picture to picture). Alsobeing considered in JCT-VC are proposals to allow “updating” an APS atslice boundaries, which, if accepted, would also make the APS anappropriate syntax structure for column width update information, asdescribed below.

In some embodiments of the invention, the column boundaries that havebeen changed through the override mechanism discussed below revert backto the locations specified at the picture or sequence level whenencoding/decoding of the current picture is complete. This feature may,at least partly, compensate for the architectural lack of cleanliness ofusing the slice header to code information pertaining to more than oneslice. For example, one possible reason for disallowing the use ofpicture headers and similar data structures in H.264 is to avoid errorpropagation due to corrupted header structures. Reverting back to dataconveyed in parameter sets (which have been designed to be verydifficult to ever get corrupted) after each picture may ensure thatheader corruption is limited to one given picture, rather to allpictures following the picture in which the header corruption hasoccurred.

Shown in FIG. 5 are two original column boundaries 501 and 502, whichare depicted as punctuated, boldface, vertical lines. In the example, itis assumed that the encoder wishes for some reason to reduce the columnwidth of the leftmost column 503. The motivation of the encoder to makethis adjustment is not generally limited and may include, for example,processor load balancing aspects, or relatively (with respect to othercolumns) high coding complexity of the content in the leftmost column,that would lead to more bits in the CTBs of this column, and, inconsequence, results in elongated slices unless the column width isreduced.

According to the modification 500, column boundary 502 spans the entirevertical height of the picture, while column boundary 501 spans only aportion of the entire vertical height of the picture, i.e., the twouppermost rows of CTBs. The remainder of the column boundary is offsetrelative to column boundary 501 and, consequently, the leftmost column503 has variable width. For example, column 503 may be split into twodifferent slices of different width. As shown in FIG. 5, slice 504within column 503 may have a first width, which is set by the originalcolumn boundary 501, whereas slice 505 within column 503 may have asecond width different from the first width, and which is set by theoffset column boundary according to the modification 500.

Column boundaries can be modified at the slice level by includingboundary offset values to shift a column boundary to, for example, theleft, if the offset value is negative, or alternatively to, for example,the right, if the offset value is positive. The slice header of theslice 505 in the leftmost column 503 may, for example, includeinformation to the extent that the original column boundary 501 betweenthe leftmost and middle columns, as defined with respect to slice 504,be shifted by one CTB to the left in respect of the slice 505. As aresult, CTB 22 belongs now to the middle column (as opposed to theleftmost column 503), and coding continues in the leftmost column 503with slice 505, which encompasses CTBs 20, 21, 30, 31, 40, 41, 50, and51 in this particular example.

There are a number of corner cases that could potentially makeimplementation of the modification 500 described above seemunnecessarily complex and/or inefficient from a caching viewpoint.However, such corner cases may be effectively handled by a number ofrestrictions concerning the moving of column boundaries that can beimposed in various embodiments, and which are described further below.

During picture decoding, shifting of the column boundary may come intoeffect starting at the CTB line in which is locate the address of thefirst CTB of the slice to contain the boundary offset information in itsslice header. Thereafter, the boundary offset may remain in effect forall remaining CTB lines in the picture unless such boundary offset isoverridden by another boundary offset in a subsequent slice header.

The boundary offset values may be indicated in units of CTB widths.

The bitstream may contain a “boundary_offset_flag” (i.e., as seen inFIG. 6), which can be parsed from the bitstream. If theboundary_offset_flag has a value of one, then one or both of aleft_boundary_offset and right_boundary_offset may follow in thebitstream through which column boundary shifts to the right or left maybe specified. Otherwise, if the boundary_offset_flag has a value ofzero, then no offset values may be present in the bitstream followingthe boundary_offset_flag. In the style of the H.264 syntaxspecification, one example syntax for specifying the offset values isshown below:

boundary _(—) offset _(—) flag if (boundary_offset_flag) { left _(—)boundary _(—) offset right _(—) boundary _(—) offset }

As above, boldface variables (i.e., lines 1, 3, and 4 above), incomparison to a plain formatted line (i.e., line 2), may represent bitsthat are fetched from the bitstream as a side effect of or otherwise inconjunction with determining the value of the variable.

FIG. 6 is a flow diagram illustrating a method 600 that may be used todetermine column boundaries based on boundary offset values parsed asabove, in accordance with embodiments of the invention.

The method 600 may be initialized (601) by the following settings, whichfor convenience are expressed in C-style pseudo-code. Suchinitialization, among other things, copies the parsed boundaries fromthe variable columnBoundary[ ], which was derived from the high levelsyntax elements described above, as follows:

pictureWidth = width of the picture // in units of CTBs colBound[0] = 0;colBound[num_column_minus_1+1]=pictureWidth; for (j=1;j<num_column_minus_1+1; j++) colBound[j]=columnBoundary[j];

Thereafter, it is determined (602) whether the boundary_offset_flag(introduced above) is set. If it is determined (602) that the flag isnot set, then updates of column boundaries are not allowed, and themethod 600 branches (603) so that the slice is decoded (604) aspreviously described and specified in the video coding standard to whichthe decoding complies, for example, which may involve, for each CTB ofthe slice, processes such as entropy decoding, motion compensation toform a predictor, inverse quantization and inverse transform ofcoefficients to calculate a residual, adding the residual to the motioncompensated predictor, in-CTB deblock-filtering the resulting samples,and so forth. Then if it is determined (605) the last slice in thepicture has been decoded (a situation that can be tested by manydifferent mechanisms, such as detecting a change in the value of thepictureID, Temporal Reference, presentation time stamp, or similarsyntax element, as well as timeout mechanisms based on RTP timestamps),then the method 600 continues (606) by initializing (601) as describedabove to process the next picture in the bitstream. However, if it isdetermined (605) that the slice was not the last one of the picture,then the method 600 continues (607) by determining (602) if theboundary_offset_flag is set.

If it is determined (602) that the boundary_offset_flag is set, then theslice header contains boundary offsets that can be parsed and used tomodify colBound[ ], and the method branches (608) for such modificationof column boundaries. The modification may commence by an identificationof the column to which the slice belongs. Such identification may inturn commence with an initialization (609), for example, which cancontain instructions such as the following:

firstCTBMod = // first CTB of current slice modulo picture Width //where first CTB is the CTB address in scan order leftOffset =left_boundary_offset; // as parsed rightOffset = right_boundary_offset;// as parsed i = 1;

At this point, the method 600 may search through the current state ofthe column boundaries (e.g., in colBound[ ]) so as to identify theparticular column to which the slice being decoded belongs. For example,the method 600 may perform such identification by checking (611) thehorizontal address component of the CTB address of the current CTB(which is the first CTB in the slice in question), as determined in theinitialization and stored in firstCTBMod, against the stored columnboundary values (colBound[ ]). By searching through the columnboundaries by incrementing (610) a suitable search index, once a columnboundary is identified as being located to the “right” (i.e., as havinga larger horizontal address in terms of scan order) of the currentslice's first CTB (as expressed in firstCTBMod), the search is finishedand the method 600 exits (612) the search loop with the surviving stateof the search index i.

The boundaries of the column to which the slice being decoded belongsmay be updated (613), according to the surviving state of the searchindex i, by adding the difference information leftOffset andrightOffset, as initialized (609), to the left and right columnboundaries Such updating may be achieved, for example, by the following:colBound[i−1]=colBound[i−1]+leftOffset;colBound[i]=colBound[i]+rightOffset;

At this point, the method 600 proceeds to the slice decoding (604) asdescribed above.

In some case, it may be advantageous and/or convenient to imposeconstraints on the column boundary indication including the followingconstraint:colBound[i]<colBound[i+1]

Such constraint requires columns to be numbered from the left to theright in the picture and also implies that offset values cannot be usedto switch column boundary order.

The leftOffset, rightOffset and subsequently derived colBound[ ] valuesmay also be constrained such that each resulting column has a widthwhich is greater than or equal to a minimum column width. For example,the minimum column width may be level specific and/or specified in thevideo compression standard.

In some cases, the leftOffset and rightOffset may not be used to modifya colBound[ ] used as a slice boundary for a previously coded slice.

In some cases, the value of colBound[0] may not be changed from itsinitial value of ‘0’ to reflect that the first numbered column begins atthe leftmost edge of the picture. Consequently, the value of leftOffsetwould be ‘0’ in such cases when i=1.

In some cases, the value of colBound[num_columns_minus1+1] may not bechanged from its initial value of pictureWidth. Consequently, the valueof rightOffset would be ‘0’ in such cases when i=num_column_minus_1+1.

Referring to FIG. 7, shown is a block diagram of a decoder 700 that isbased on a multiprocessor or multicore architecture in accordance withthe described embodiments. For convenience, the decoder 700 is assumedto operate on a bitstream configured or made suitable for paralleldecoding, for example, by requiring the use of columns with certainlimitations in column width, as already described (e.g., upper and lowerlimits on column width).

In some embodiments, the decoder 700 can receive a coded bitstream 701into a high level syntax interpreter 702. The bitstream 701 is shown inFIG. 7 as a punctuated line to distinguish the bitstream 701 frommetadata (shown as solid hairline) and from sample data (shown in solidboldface line). The high level syntax interpreter 702 can be configuredfor decoding of parameter sets and subsequent storage in a parameter setdatabase 703 to which the high level syntax interpreter 702 has read andwrite access. The decoded parameter sets may be stored and provided 704to one or more other components in the decoder 700, for example, to acolumn/slice decoder 705. In some embodiments, the high level syntaxinterpreter 702 may be capable of further handling parts of, or all of,the entropy decoding of high level syntax and/or low level syntax (belowCU level) elements included in the bitstream 701, which capability maydepend on the assignment of entropy decoding to the column decoders 707,708. The output data 706 of the high level syntax interpreter 702 canbe, for example, metadata components derived from the bitstream 701 thatcan include information from the bitstream, such as symbols (createdthrough entropy decoding) or buffers containing entropy coded symbols,as well as control information.

The column/slice decoder 705 may be coupled to the high level syntaxinterpreter 702 and to the parameter set database 703, and may receivethe output data 706 from the high level syntax interpreter 702. In someembodiments, the column/slice decoder 705 may decode slice headers, andidentify the column to which the first CTB of the slice belongs, forexample, by taking into account column boundary information that can belocated in parameter sets, stored in the parameter set database 703, andretrieved 704 based on the parameter set reference in the slice header.If the slice/column decoder 705 determines that a slice resides in asingle column, as described above, then the column/slice decoder 705 mayidentify the column in which the slice resides by identifying the columnto which the first CTB of the slice belongs.

Based on the column information, the column/slice decoder 705 may selectone column decoder for the decoding of the slice. Two column decoders707, 708 are shown in FIG. 7 as an example implementation of the decoder700 and are not intended to be limiting of the possible implementations.For example, in alternatively embodiments, the encoder 700 may include agreater, or fewer, numbers of column decoders, provided the encoder 700includes at least one column decoder. If there are more columns in thebitstream 701 than column decoders 707, 708 included in the decoder 700,then at least one of the column decoders 707, 708 may be selected forthe decoding of more than one column. Alternatively, if there are morecolumn decoders 707, 708 available in the decoder 700 than there arecolumns in the bitstream 701, then one or more of the column decoders707, 708 may be idle as no column will be assigned to them by thecolumn/slice decoder 705. However the column/slice decoder 705 mayassign columns in the bitstream 701 to the column decoders 707, 708 maybe handled by the column/slice decoder 705 as a load balancing problemfor which many possible solutions may be apparent after familiarizationwith the present disclosure.

Assuming that the slice currently to be decoded is assigned to columndecoder 707, data 709 required to decode the column can be forwarded tothe column decoder 707. Such data 709 may have similar characteristicsas output data 706 from the high level syntax interpreter 702. In someimplementations, the column/slice decoder 705 may prune, or mark asirrelevant, those parts of the output data 706 that are not required forthe decoding of the column, with the effect that such unneeded parts arenot forwarded to column decoder 707 as part of data 709.

In some embodiments, the bitstream 701 may be configured or mayotherwise allow for completely or near completely independent decodingof its content in multiple column decoders (e.g., 707 and 708 in FIG.7). However, in other cases, some metadata or sample data informationmay need to be shared between multiple column decoders. Further detailsrelevant to the nature and amount of data that may potentially need tobe shared between multiple column decoders may be found in co-pendingU.S. patent application Ser. No. 13/336,475.

In some embodiments, sharing of information can be handled by a suitablyconfigured sharing unit 710. The sharing unit 701 may be implemented asshared memory, a queuing mechanism that allows message exchange, orother mechanisms for inter-processor or inter-core communication. Whereinformation sharing is enabled by sharing unit 710, it may potentiallybe necessary to share both meta-information 711 (e.g., entropy decodingstate, motion vector predictors) and/or sample data 712 (e.g., values ofneighbouring pixels of the column used for intra prediction or motioncompensation) between the column decoders 707, 708. The sharing unit 710may also be suitably configured so as to facilitate the “handover” ofinformation from one column decoder 707, 708 to another in such caseswhere a slice spans across a column boundary between adjacent columns,with the effect that multiple column decoders 707, 708 would be assignedin turn by the slice/column decoder 705 to decode such a slice.

In some embodiments, for example, in implementations of the decoder 700that utilize a multi-processor or single multicore processor in apersonal computer or similar hardware architecture, the implementationof the column decoders 707, 708 as processes running on cores orprocessors can be greatly facilitated by keeping all state informationof the decoder 700, including for example full reference pictures, inmemory that is shared between the column decoders 707, 708. In thiscase, the sharing unit 710 may include a shared memory aided in somecases possibly by interprocessor communication tools such as semaphoresor message passing implemented, for example, in an operating systemkernel.

The output of the column decoder 708 can be partly reconstructed samples713 and associated metadata 714 provided to a combiner 715. Likewise thecolumn decoder 707 may produce partly reconstructed samples andassociated metadata, similar to partly reconstructed samples 713 andassociated metadata 714 produced by column decoder 708. The samples 713may be described as “partly” reconstructed because for fullreconstruction, as specified in the video compression standard, furtherprocessing may be required in the picture-based sample generator 717, asdescribed later. The samples 713 and associated metadata output 714provided by column decoder 708 may be combined in the combiner 715 withthe corresponding outputs of the column decoder 707 to generate acolumn-processed picture 716 and associated metadata 719.

The picture-based sample generator 717 may be configured so as togenerate the final reconstructed picture 718 based on thecolumn-processed picture 716 and associated metadata 719, which isreceived from the combiner 715. The final reconstructed picture 718 maybe used possibly for output by the decoder 700 and/or as a referencepicture that is fed back to other components within the decoder 700(noted further below). The picture-based sample generator 717 canadvantageously implement tasks that are not necessarily verycomputationally intensive, but which require access to informationrelating to all columns. In-loop filtering, post filtering, and errorconcealment are examples of those tasks. However, depending on thecharacteristics of the encoded bitstream 701, it may also be possiblethat some or all of these tasks can be implemented instead in the columndecoders 707, 708 (further details of which may be found in co-pendingU.S. patent application Ser. No. 13/336,475). Therefore, in someembodiments, the picture-based sample generator 717 may be omitted fromthe decoder 700.

Not specifically shown in FIG. 7 is the “feedback” of the referencepicture samples and/or metadata to a location where the columndecoder(s) 707, 708 have access to the information contained within thereference picture samples and/or metadata that be of relevance. In someembodiments, the column decoder(s) 707, 708 may be provided withassociated reference picture memory, in which feedback of the referencepicture samples and/or metadata may not be required. In otherembodiments, the combiner 715 can may provided with a reverse data paththat allows for reference picture data from the picture-based samplegenerator 717 to be fed back to the column decoder(s) 707, 708. In FIG.7, such feedback path is indicated by the double-ended tips of thearrows 713, 714, 716, and 719. In still other embodiments, the referencepictures may be stored in the sharing unit 710 (in which case acorresponding data path could be provided between picture-based samplegenerator 717 and sharing unit 710, but not shown in FIG. 7 to enhanceclarity).

FIG. 8 is a block diagram illustrating a data processing system (e.g., apersonal computer (“PC”)) 800 based implementation in accordance with anembodiment of the invention. Up to this point, for convenience thedescription has not related explicitly to possible physicalimplementation of the encoder and/or decoder in detail. Many differentphysical implementations based on combinations of software and/orcomponents are possible. For example, in some embodiments, the videoencoder(s) and/or decoder(s) may be implemented using custom or gatearray integrated circuits, in many cases, for reasons related to costefficiency and/or power consumption efficiency.

Additionally, software based implementations are possible using generalpurpose processing architectures, an example of which is the dataprocessing systems 800. For example, using a personal computer orsimilar device (e.g., set-top-box, laptop, mobile device), such animplementation strategy may be possible as described in the following.As shown in FIG. 8, according to the described embodiments, the encoderand/or the decoder for a PC or similar device 800 may be provided in theform of a computer-readable media 801 (e.g., CD-ROM, semiconductor-ROM,memory stick) containing instructions configured to enable a processor802, alone or in combination with accelerator hardware (e.g., graphicsprocessor) 803, in conjunction with memory 804 coupled to the processor802 and/or the accelerator hardware 803 to perform the encoding ordecoding. The processor 802, memory 804, and accelerator hardware 803may be coupled to a bus 805 that can be used to deliver the bitstreamand the uncompressed video to/from the aforementioned devices. Dependingon the application, peripherals for the input/output of the bitstream orthe uncompressed video may be coupled to the bus 805. For example, acamera 806 may be attached through a suitable interface, such as a framegrabber 807 or a USB link 808, to the bus 805 for real-time input ofuncompressed video. A similar interface can be used for uncompressedvideo storage devices such as VTRs. Uncompressed video may be outputthrough a display device such as a computer monitor or a TV screen 809.A DVD RW drive or equivalent (e.g., CD ROM, CD-RW Blue Ray, memorystick) 810 may be used to input and/or output the bitstream. Finally,for real-time transmission over a network 812, a network interface 811can be used to convey the bitstream and/or uncompressed video, dependingon the capacity of the access link to the network 812, and the network812 itself.

According to various embodiments, the above described method(s) may beimplemented by a respective software module. According to otherembodiments, the above described method(s) may be implemented by arespective hardware module. According to still other embodiments, theabove described method(s) may be implemented by a combination ofsoftware and hardware modules.

While the embodiments have, for convenience, been described primarilywith reference to an example method, the apparatus discussed above withreference to a data processing system 800 may, according to thedescribed embodiments, be programmed so as to enable the practice of thedescribed method(s). Moreover, an article of manufacture for use with adata processing system 800, such as a pre-recorded storage device orother similar computer readable medium or product including programinstructions recorded thereon, may direct the data processing system 800so as to facilitate the practice of the described method(s). It isunderstood that such apparatus and articles of manufacture, in additionto the described method(s), all fall within the scope of the describedembodiments.

In particular, the sequences of instructions which when executed causethe method described herein to be performed by the data processingsystem 800 can be contained in a data carrier product according to oneembodiment of the invention. This data carrier product can be loadedinto and run by the data processing system 800. In addition, thesequences of instructions which when executed cause the method describedherein to be performed by the data processing system 800 can becontained in a computer program or software product according to oneembodiment of the invention. This computer program or software productcan be loaded into and run by the data processing system 800. Moreover,the sequences of instructions which when executed cause the methoddescribed herein to be performed by the data processing system 800 canbe contained in an integrated circuit product (e.g., a hardware moduleor modules) which may include a coprocessor or memory according to oneembodiment of the invention. This integrated circuit product can beinstalled in the data processing system 800.

The embodiments of the invention described herein are intended to beexemplary only. Accordingly, various alterations and/or modifications ofdetail may be made to these embodiments, all of which come within thescope of the invention.

What is claimed is:
 1. A method for decoding in a decoder a bitstream ofcoded pictures, the method comprising: obtaining, by one or moreprocessors and from a first picture parameter set encoded in thebitstream, a value for a number of columns N minus one for a first codedpicture of the bitstream, the first coded picture comprising a pluralityof coded tree blocks (CTBs), the first coded picture segmented into aplurality of slices and a plurality of columns, wherein a first slice ofthe plurality of slices spans multiple columns of the plurality ofcolumns; determining, by the one or more processors, a first CTB sizefor the first coded picture from among a plurality of different CTBsizes; obtaining, by the one or more processors and from the firstpicture parameter set encoded in the bitstream, a signal indicatingwhether column widths pertaining to a leftmost N−1 columns of the firstcoded picture are equal; determining, by the one or more processors andbased on the signal, a column width, measured in units of the first CTBsize, for each of the leftmost N−1 columns of the first coded picture;determining, by the one or more processors, a width of the first codedpicture measured in units of the first CTB size; calculating, by the oneor more processors, a width of a rightmost column of the first codedpicture using the width of the first coded picture and the determinedcolumn widths pertaining to the leftmost N−1 columns of the first codedpicture, wherein, when the signal obtained from the first pictureparameter set encoded in the bitstream indicates that the column widthspertaining to the leftmost N−1 columns of the first coded picture areequal, the calculated width of the rightmost column of the first codedpicture is not equal to the column width of a leftmost column of thefirst coded picture; decoding, by the one or more processors, the firstcoded picture using the calculated width of the rightmost column of thefirst coded picture and the determined column width for the leftmost N−1columns of the first coded picture to generate a reconstructed picture,comprising: determining, for each of the CTBs in the first codedpicture, a respective column to which the CTB belongs using thecalculated width of the rightmost column of the first coded picture andthe determined column width for the leftmost N−1 columns, and for eachslice in the first coded picture, decoding the CTBs in the slice in anorder that is based on the respective columns to which the CTBs in theslice belong, comprising during decoding of the first slice, breakingin-loop filtering within the first slice at each column boundary betweenadjacent columns of the multiple columns spanned by the first slice,wherein the CTBs in the slice are decoded in an ordered sequence, firstby CTB from left to right and top to bottom across all CTBs within afirst column of the respective columns, and second by column from leftto right across all the columns in the slice; and outputting, by the oneor more processors, the reconstructed picture.
 2. The method of claim 1,wherein the signal indicates that the column widths pertaining to theleftmost N−1 columns are equal, and wherein determining, based on thesignal, a column width for each of the leftmost N−1 columns of the firstcoded picture comprises calculating, based on the first pictureparameter set, equal column widths pertaining to leftmost columnsmeasured in units of the first CTB size.
 3. The method of claim 2wherein the width of the rightmost column for the first coded picture isunequal to the width of each of the leftmost N−1 columns.
 4. The methodof claim 1 further comprising: determining, from a second pictureparameter set, a column width for each of the leftmost N−1 columnsmeasured in units of a second CTB size for a second coded picture of thebitstream, the second CTB size differing from the first CTB size.
 5. Themethod of claim 1, wherein the plurality of CTBs of the first codedpicture are associated as slices, each of the slices residing entirelywithin one column.
 6. The method of claim 1, wherein the plurality ofdifferent CTB sizes includes 32×32, 16×16, and 8×8 samples.
 7. Themethod of claim 1, wherein the rightmost column for the first codedpicture spans the entire vertical height of the first coded picture. 8.The method of claim 1 further comprising: calculating, based on a secondpicture parameter set, equal column widths pertaining to leftmostcolumns measured in units of a second CTB size for a second codedpicture of the bitstream, the second CTB size differing from the firstCTB size.
 9. The method of claim 1, wherein the signal does not indicatethat the column widths pertaining to the leftmost N−1 columns are equal,and wherein determining, based on the signal, a column width for each ofthe leftmost N−1 columns of the first coded picture comprises obtaining,from the first picture parameter set, a column width for each of theleftmost N−1 columns of the first coded picture, the column widths beingmeasured in units having the first CTB size.
 10. A method for decodingin a decoder a bitstream of coded pictures, the method comprising:obtaining, by one or more processors and from a first picture parameterset encoded in the bitstream, a value for a number of columns N minusone for a first coded picture of the bitstream, the first coded picturecomprising a plurality of coded tree blocks (CTBs), the first codedpicture segmented into a plurality of slices and a plurality of columns,wherein a first slice of the plurality of slices spans multiple columnsof the plurality of columns; obtaining, by the one or more processorsand from the first picture parameter set encoded in the bitstream, asignal indicating that column widths pertaining to an N−1 leftmostcolumns of the first coded picture are uniform; determining, by the oneor more processors, a width of the first coded picture measured in unitsof the first CTB size, the first CTB size being determined from aplurality of sizes; calculating, by the one or more processors, a width,measured in units of the first CTB size, for each column of the firstcoded picture using the width of the first coded picture and the numberof columns N of the first coded picture, wherein the calculated widthfor the rightmost column for the first coded picture is not equal to thecalculated width for a leftmost column of the first coded picture;decoding, by the one or more processors, the first coded picture usingthe calculated widths of the columns in the first coded picture togenerate a reconstructed picture, comprising: determining, for each ofthe CTBs in the first coded picture, a respective column to which theCTB belongs using the calculated widths of the columns of the firstcoded picture, and for each slice in the first coded picture, decodingthe CTBs in the slice in an order that is based on the respectivecolumns to which the CTBs in the slice belong, comprising duringdecoding of the first slice, breaking in-loop filtering within the firstslice at each column boundary between adjacent columns of the multiplecolumns spanned by the first slice, wherein the CTBs in the slice aredecoded in an ordered sequence, first by CTB from left to right and topto bottom across all CTBs within a first column of the respectivecolumns, and second by column from left to right across all the columnsin the slice; and outputting, by the one or more processors, thereconstructed picture.
 11. The method of claim 10 wherein the width ofthe rightmost column for the first coded picture is unequal to the widthof each of the leftmost N−1 columns.
 12. The method of claim 10, whereinthe plurality of CTBs of the first coded picture are associated asslices, each of the slices residing entirely within one column.
 13. Themethod of claim 10, wherein the plurality of different CTB sizesincludes 32×32, 16×16, and 8×8 samples.
 14. The method of claim 10,wherein the rightmost column for the first coded picture spans theentire vertical height of the first coded picture.
 15. A decoder fordecoding a bitstream of coded pictures, the decoder comprising: one ormore computers and one or more storage devices storing instructions thatare operable, when executed by the one or more computers, to cause theone or more computers to perform operations comprising: obtaining, froma first picture parameter set encoded in the bitstream, a value for anumber of columns N minus one for a first coded picture of thebitstream, the first coded picture comprising a plurality of coded treeblocks (CTB), the first coded picture segmented into a plurality ofslices and a plurality of columns, wherein a first slice of theplurality of slices spans multiple columns of the plurality of columns;determining a first CTB size for the first coded picture from among aplurality of different CTB sizes; obtaining, from the first pictureparameter set encoded in the bitstream, a signal indicating whethercolumn widths pertaining to leftmost N minus 1 columns of the firstcoded picture are equal; determining, based on the signal, a columnwidth, measured in units of the first CTB size, for each of the leftmostN−1 columns of the first coded picture; determining a width of the firstcoded picture measured in units of the first CTB size; calculating awidth of a rightmost column of the first coded picture using the widthof the first coded picture and the determined column widths pertainingto the leftmost N−1 columns of the first coded picture, wherein, whenthe signal indicates that the column widths pertaining to the leftmostN−1 columns of the first coded picture are equal, the calculated widthof the rightmost column of the first coded picture is not equal to thecolumn width of a leftmost column of the first coded picture; decoding,by the one or more processors, the first coded picture using thecalculated width of the rightmost column of the first coded picture andthe determined column width for the leftmost N−1 columns of the firstcoded picture to generate a reconstructed picture, comprising:determining, for each of the CTBs in the first coded picture, arespective column to which the CTB belongs using the calculated width ofthe rightmost column of the first coded picture and the determinedcolumn width for the leftmost N−1 columns, and for each slice in thefirst coded picture, decoding the CTBs in the slice in an order that isbased on the respective columns to which the CTBs in the slice belong,comprising during decoding of the first slice, breaking in-loopfiltering within the first slice at each column boundary betweenadjacent columns of the multiple columns spanned by the first slice,wherein the CTBs in the slice are decoded in an ordered sequence, firstby CTB from left to right and top to bottom across all CTBs within afirst column of the respective columns, and second by column from leftto right across all the columns in the slice; and outputting thereconstructed picture.
 16. The decoder of claim 15, wherein the signalindicates that the column widths pertaining to the leftmost N−1 columnsare equal, and wherein determining, based on the signal, a column widthfor each of the leftmost N−1 columns of the first coded picturecomprises calculating, based on the first picture parameter set, equalcolumn widths pertaining to leftmost columns measured in units of thefirst CTB size.
 17. The decoder of claim 16 wherein the width of therightmost column for the first coded picture is unequal to the width ofeach of the leftmost N−1 columns.
 18. The decoder of claim 15, whereinthe operations further comprise: determining, from a second pictureparameter set, a column width for each of the leftmost N−1 columnsmeasured in units of a second CTB size for a second coded picture of thebitstream, the second CTB size differing from the first CTB size. 19.The decoder of claim 15, wherein the plurality of CTBs of the firstcoded picture are associated as slices, each of the slices residingentirely within one column.
 20. The decoder of claim 15, wherein theplurality of different CTB sizes includes 32×32, 16×16, and 8×8 samples.